The concept of using monolithic honeycomb structures to provide solid oxide fuel cells (SOFC) is well known. U.S. Pat. Nos. 4,476,198 and 4,666,798, for example, describe fuel cell assemblies built up in complex fashion from honeycomb cell segments incorporating contacting electrolyte, anode, cathode, and interconnect layers that cooperate to impart the necessary strength and structural integrity to the completed cell. Alternate channels of the honeycomb monolith are fed with fuel and oxidant (e.g., air) respectivelyxe2x80x94yielding a large current-generating active area. Anticipated advantages of honeycomb configurations include an increase in volume power density (power per unit of fuel cell volume).
Common features of many of these prior art fuel cell designs are cell segments based on an interconnecting honeycomb or sheet framework of electrolyte material onto or within which the electrodes are subsequently deposited or attached. One difficulty with this approach is that neither the electrolyte framework nor the attached electrodes alone impart any real structural strength to the cell assembly. At most these structures have been self-supporting, and relatively susceptible to mechanical damage from even the thermal stresses regularly encountered in use.
Because good SOFC performance requires low electrolyte resistance, especially at lower temperatures (below 800xc2x0 C.) where low cost metallic components may be employed, the use of thicker stronger electrolyte honeycomb structural members is not a viable remedy. In the case of yttria-stabilized zirconia, for example, which is a favored SOFC electrolyte, an electrolyte layer thickness of less than about 50 microns will probably be needed for acceptable fuel cell performance. The fabrication of honeycombs having defect-free walls of such thickness, especially by means of economic forming processes such as direct honeycomb extrusion, is not presently feasible. Hence alternative honeycomb fuel cell designs, especially if enabled by inexpensive forming processes not requiring the direct formation of thin-walled electrolyte structures, would be desirable.
In accordance with the invention a solid oxide fuel cell design based on a fuel cell electrode of honeycomb structure is provided. The honeycomb electrode, which may be the anode or the cathode of the fuel cell, incorporates gas-permeable interconnecting walls forming parallel channels extending from one face to the other of the honeycomb. The channels are open-ended to allow for the free flow of fuel or oxidant gases therethrough.
The electrolytes and counter-electrodes in these fuel cell designs are provided as coatings within selected, e.g., alternate, channels of the honeycomb structure. That is, plural electrolyte layers are provided on the walls of selected channels within the honeycomb, and counter-electrode layers are deposited on top of those electrolyte layers to form the electrode/electrolyte/counter-electrode structure required for fuel cell operation.
When the honeycomb electrode is to function as the anode or fuel side of the fuel cell, those honeycomb channels selected to carry the oxidant for the cell will be provided with electrolyte and cathode layers, while the alternate or anode channels will be left uncoated. As in conventional honeycomb fuel cell designs, the cathode or oxidant channels will share gas-permeable channel walls with the anode or fuel channels to allow the fuel-oxidant reactions to occur at the electrolyte.
Honeycomb electrode fuel cells provided in accordance with the invention offer significant electrical and mechanical advantages over conventional fuel cells of honeycomb structure. The gas-permeable honeycomb electrodes can be as heavy as needed to provide the required mechanical durability for the cell without compromising electrical performance. The use of a ductile metal or metallic honeycomb electrode structure substantially improves the thermal shock tolerance of the structure, and the use of extrusion processes to form the honeycomb electrodes, whether of metallic or ceramic structure, provides a significant cost advantage. Finally the significant electrochemical performance advantages of extremely thin electrolyte layers can be fully realized since the electrolytes are not required to provide any structural support whatever to the fuel cell assembly.